When encountering an Item in a pat context, point at the item def

```
error[E0308]: mismatched types
  --> $DIR/const-in-struct-pat.rs:8:17
   |
LL | struct foo;
   | ----------- `foo` defined here
...
LL |     let Thing { foo } = t;
   |                 ^^^ expected struct `std::string::String`, found struct `foo`
   |
   = note: `foo` is interpreted as a unit struct, not a new binding
help: you can bind the struct field to a different name
   |
LL |     let Thing { foo: other_foo } = t;
   |                 ^^^^^^^^^^^^^^
```
```
error[E0308]: mismatched types
  --> $DIR/const.rs:14:9
   |
LL | const FOO: Foo = Foo{bar: 5};
   | ----------------------------- constant defined here
...
LL |         FOO => {},
   |         ^^^
   |         |
   |         expected `&Foo`, found struct `Foo`
   |         `FOO` is interpreted as a constant, not a new binding
   |         help: use different name to introduce a new binding: `other_foo`
```

Fix rust-lang#55631, fix rust-lang#48062, cc rust-lang#42876.

Rollup of 9 pull requests

Successful merges:

 - #67741 (When encountering an Item in a pat context, point at the item def)
 - #68985 (Parse & reject postfix operators after casts)
 - #69656 (Use .next() instead of .nth(0) on iterators.)
 - #69680 (rustc_expand: Factor out `Annotatable::into_tokens` to a separate method)
 - #69690 (test(pattern): add tests for combinations of pattern features)
 - #69706 (Use subslice patterns in slice methods)
 - #69727 (Avoid using `unwrap()` in suggestions)
 - #69754 (Update deprecation version to 1.42 for Error::description)
 - #69782 (Don't redundantly repeat field names (clippy::redundant_field_names))

Failed merges:

r? @ghost